In recent years, the annual water quality assessment of Taihu Lake shows that 85.2% of the monitoring section water quality cannot meet Grade III requirements, most of them are Grade V and poor Grade V. The main exceeded pollutants are ammonia nitrogen, permanganate index, dissolved oxygen, biochemical oxygen demand after 5 days (BOD5), petroleum, total phosphorus and chemical oxygen demand. The water quality of the upper stream river inflowing into lake and the lake area continued to deteriorate, directly leading to an increase in the total amount of pollutants in the Taihu Lake. The contents of TP and TN in the Taihu Lake were increasing in recent years due to the increase of inflowing into lake pollutants. Therefore, the key to protect the water environment of Taihu Lake is to intercept the pollutants from the source inflowing into Taihu Lake, and control the total amount of pollutants discharged into the lake area from upper stream river.
At present, there are nearly 200 urban sewage treatment plants in the Taihu Lake, all of them are executing the Grade 1 (A) discharge standard in “Pollutant Discharge Standards for Urban Sewage Treatment Plants” (GB18918-2002). But the concentration of nitrogen and phosphorus in the Grade 1 (A) discharge standard (TN 15 mg/L, NH3—N5 (8) mg/L, TP 0.5 mg/L) still far exceeds the surface water environmental quality standard (surface water Grade V water standard TN 2 mg/L, NH3—N 2 mg/L, TP 0.4 (lake 0.2) mg/L). Facing the increasingly serious eutrophication of Taihu Lake, if tail water directly be discharged from sewage treatment plant without further treatment, it will have a big impact on the water quality of the rivers channel inflowing into the lake, then it will aggravate the pollution degree of nitrogen and phosphorus in Taihu Lake, threatening the safety of drinking water. At the same time, carrying out the further treatment and reuse of tail water is also a powerful measure to solve the problem of water shortage in Taihu Lake. It has significant environmental, economic and social benefits.
At present the commonly used water further treatment technologies are mainly physic-chemical method (filtration, adsorption, etc.), biological method (bioreactor, biofilter, constructed wetlands, etc.) and membrane separation method (reverse osmosis, microfiltration, nanofiltration, etc.). The constructed wetlands technology is widely used because of its low investment and maintenance cost, good effect of removing nitrogen and phosphorus, small secondary pollution and both scenery and view effects.
It was found that the nitrogen was highly nitrified and the carbon source was seriously insufficient in sewage treatment plant tail water. In addition, the carbon source of the water inlet was insufficient in nearly 50% of the urban sewage treatment plant in the Taihu Lake. However, the carbon source is the electron donor in the process of denitrification, which is the key factor for restricting the denitrification. To achieve the further treatment of the tail water of the sewage treatment plant, enough additional carbon source must be added to ensure a certain ratio of carbon and nitrogen, and then the denitrification process can be completed successfully.
The traditional denitrification carbon sources include glucose, methanol, ethanol and acetic acid, etc. But these carbon sources are expensive, and some of them, such as methanol, ethanol, acetic acid, etc have a certain toxicity and have a potential risk to the environment. In recent years, many researchers domestic and overseas try to find a new carbon source with low toxicity and cost to replace the traditional carbon source.
A large number of aquatic plants which are rich in cellulose matter are planted in constructed wetlands, and these plants can produce large amounts of volatile fatty acids (VFAs) and other nutrients by anaerobic fermentation, which are excellent potential additional carbon source of denitrification.
Taking the aquatic plants planted in constructed wetlands as raw materials, the cellulose matter in the plants is converted into volatile fatty acids (VFAs) and other nutrient elements by anaerobic fermentation, used to be the carbon source of denitrification. Results showed that further nitrogen removal of sewage treatment plant tail water is achieved, and the resource utilization of aquatic plants is realized at the same time.
The previous research results showed that the nitrification and denitrification of microorganisms are important ways of nitrogen cycling in nature. Denitrification is the process that under anaerobic or hypoxic conditions microorganism converts the nitrate nitrogen and nitrite nitrogen into nitrogen and release it into the atmosphere. The main influencing factors of nitrogen removal are dissolved oxygen (DO), pH, temperature, carbon source, etc.
(1) Dissolved Oxygen (DO): in order to ensure normal denitrification, dissolved oxygen must be kept at 0.5 mg/L or below. This is because the ability of O2 to accept electrons is stronger than that of NO2—N and NO3−—N. When both molecular oxygen and nitrate are existed, denitrifying bacteria preferentially carry out aerobic respiration.
(2) pH: the optimal pH of denitrification is 7-8.
(3) Temperature: the optimal temperature of denitrification is 15˜30. Denitrifying bacteria are more sensitive to temperature reduction than nitrifying bacteria. When seasonal cooling occurs, the denitrification process will be inhibited before the nitrification process, at this time additional carbon source is needed in order to improve the denitrification effect. In addition, the temperature has a significant impact on the mircrobial activity, and then affecting the effect of denitrification.
(4) Carbon source: carbon source is the electron donor in the denitrification process, and it is also the main source of energy for microbial growth and reproduction. The lack of carbon source will directly affect the denitrification. Adding additional carbon source is one of the effective methods to improve denitrification nitrogen removal efficiency. The species and the amount of the additional carbon source will have a significant impact on denitrification efficiency.
Existing additional carbon sources can be broadly divided into two categories, the traditional carbon sources and the new carbon sources. Traditional carbon sources are mainly liquid state organic matter, including low-molecular organic matter (such as methanol, ethanol and acetic acid, etc.) and carbohydrate matter (such as glucose, sucrose, etc.). The new carbon sources mainly include natural solid organic matter rich in cellulose matter (such as plant stalks, etc.), some degradable artificial materials (such as waste paper, degradable lunch boxes, etc.) and high carbon content of industrial waste water.
Methanol, ethanol, acetic acid and other low molecular organic matter are easily used by denitrification bacteria, and these materials are considered as ideal additional carbon source. Gersberg et al. (1983) achieved a 95% nitrogen removal efficiency by adding methanol to the constructed wetlands system. The research results of Pochana et al. (1999) showed that the addition of acetic acid as carbon source can greatly improve the progress of simultaneous nitrification and denitrification. Rustige et al. (2007) added acetic acid as the carbon source to treat the landfill leachate in the horizontal stream section of the composite flow constructed wetlands, the results showed that the denitrification rate increased with the increasing of acetic acid concentration, and the nitrate removal rate was up to 98%. The denitrification efficiency of this species carbon source is high, but the cost is expensive and methanol has a certain toxicity and its transportation is inconvenient.
Carbohydrate matter as an additional carbon source of denitrification, the cost is lower. Zhao Lianfang et al. (2006) treated urban polluted river by constructed wetlands, the results showed that the addition of glucose could effectively improve the removal efficiency of nitrogen, when the wetlands C/N was increased from 2 to 8, TN removal rate was increased from 55% to 89%. She Lihua et al. (2009) added carbon source through specific breather pipe of the composite integrated vertical flow constructed wetlands (IVCW) system to the bottom of wetlands in order to strengthen wetlands denitrification effect. The results showed that glucose was better than carboxymethyl cellulose (CMC) as the additional carbon source, and the optical dosage of glucose was 1.5 g for integrated vertical flow constructed wetlands (IVCW) system with 60 L/d treatment capacity. Under this circumstance, the mass ratio of glucose to nitrate nitrogen was only 4.3, much lower than the ratio that denitrification required. However, when glucose was used as the carbon source, the productivity rate of microbial cells was high, which may lead to clogging of artificial wetlands and other process.
Liu Gang et al. (2010) believed that denitrification efficiency was restricted by the low-molecular organic matter content in industrial waste water when industrial waste water was used as an additional carbon source, if the low-molecular organic matter content was low, denitrification efficiency would not be significantly improved. At the same time, the dosage of industrial waste water must be controlled to prevent water quality deterioration of water outlet.
Cellulose carbon sources come from a wide range and the cost is low. At present many scholars have studied the potential implications of waste paper, corn stalks, wheat straw, straw and cattail, reed and other aquatic plant branches or stalks as carbon sources. Wenhui et al. (2011) studied the effect of wheat straw as an additional carbon source on the removal of nitrogen in constructed wetlands. The results showed that when the concentration of water inlet nitrate nitrogen was 30 mg/L, the optimal conditions for removal of nitrate nitrogen were 25, the reaction time was 10 h, the mass ratio of straw to water was 1:50. Scanning electron microscopy showed that the surface of the reacted wheat straw appeared hollow, from the dense striated structure into a broken filamentous structure, indicating that the biodegradable components of wheat straw surface were largely decomposed by microbes as denitrifying carbon source. Jin Zanfang et al. (2004) studied the nitrogen removal effect of cotton and paper as carbon sources. The results showed that both carbon sources could make the reactor start quickly. At room temperature 25, the water inlet nitrate nitrogen were 22.6 and 45.2 mg/L and hydraulic retention time were 9.8 and 8.6 h respectively, the removal rates of nitrate nitrogen were 100% and 99.6%, respectively, and no nitrite accumulation in water outlet. Chen Yunfeng et al. (2010) compared the nitrogen removal effect of wheat straw, peanut shells, sweet potato stem, corn cob, Canna litter, degradable meal boxes, polybutylene succinate (PBS) and polyhydroxyalkanoates (PHAs) as carbon sources, and the results showed that wheat straw was more suitable as the additional carbon source of denitrification for the sewage treatment plants tail water. Zhao Lianfang et al. (2009) determined that the reed rods was the more suitable plant carbon source compared to com stover, rice husk, sawdust, according to their organic matter release ability and the potential effect on water quality. When the addition amount was 1.0 kg/m2, the removal rate of TN in integrated vertical flow constructed wetlands increased from 60% to 80%. The application of cellulose matter on carbon source of denitrification could not only improve the removal efficiency of nitrogen, but also achieve the purpose of waste utilization. But its shortcoming is that the release of carbon source cannot be effectively controlled, the required hydraulic retention time is long, and the water outlet quality is susceptible to external temperature.
The urban organic waste water (such as vintage waste water, molasses waste water, starch waste water, etc.) and excess sludge in urban sewage plant contains a large number of easily biodegradable matter. After anaerobic fermentation, it can produce large amounts of short chain volatile fatty acids, such as acetic acid, propionic acid, which can be used by denitrifying microorganisms. Table 1 summarizes the nitrogen removal effects of fermentation broth of several urban organic wastes as denitrification additional carbon source.
TABLE 1Research Status of the Fermentation Broth of Abandoned Biomass as Denitrification Carbon SourceAcid-producingProduction composition/%quantity/mgCOD ·AcetivPropionicButyricDenitrification efficiency/MatrixL−1acidacidacidVFA/SCODmgNO3−—N · (gVSS · h)−1Excess sludge 92~370———0.1~0.22.4fermentationPrimary sludge3500~87004136180.69~0.942.34fermentationHydrolysis of100~2002.9~3.6molassesHydrolysis of11655423310.720.9starch waste waterHydrolysis of—————41primary sludgeFood waste water95002514180.338.2fermentationmatter
At present, most of the domestic and foreign scholars studied the nitrogen removal effect by using the anaerobic fermentation acidification products of the excess sludge in urban sewage treatment plant as the additional carbon source. The excess sludge is used as the fermentation substrate, which reduced the amount of sludge and the cost of sludge treatment, and provided high quality carbon source for nitrogen and phosphorus removal in sewage. Tong Juan (2008) used the fermentation broth of the excess sludge obtained under the alkaline condition as the additional carbon source to treat the low COD (Chemical Oxygen Demand) domestic wastewater, and used the actual sewage as the carbon source for comparative study. The results showed that in the SBR system added fermentation broth, the nitrogen and phosphorus removal rates improved a lot, and the removal of COD (Chemical Oxygen Demand), TN and SOP were 93%, 80.9% and 97.2%, respectively. When adding the actual sewage as carbon source, the removal rates of COD, TN and SOP were 85%, 63.5% and 43.9% respectively. Liu Daoguang used surfactant to promote acid production process, and then use the fermentation broth as the carbon source of nitrogen and phosphorus removal system. Results showed that the removal rates of TP, NH3—N and TN reached 97%, 95% and 81%, respectively and the VFAs in the fermentation broth was used in the sequences of butyric acid, propionic acid, acetic acid.
Potamogeton crispuses have strong vital force, wide adaptability, and thus a lot of cultivation in the constructed wetlands. Potamogeton crispuses is rich in cellulose matter. After harvest, Potamogeton crispus may produce a large amount of volatile fatty acids (VFAs) and other nutrients by anaerobic fermentation. It is an excellent potential additional carbon source and can be used as a carbon source supplement for denitrification. The further denitrification treatment of the subsurface flow type constructed wetlands with the sewage plant tail water can be realized, and the resource utilization of the aquatic plants can be realized.